[3.14] gh-116946: fully implement GC protocol for zlib objects (GH-138290)

[picnixz]

(cherry picked from commit 2a54acf)

• Issue: Heap types implemented in C must have Py_TPFLAGS_HAVE_GC and implement tp_traverse #116946

[picnixz]

Ok, so this one is correct:

• compobject is not an immutable type so it requires the GC protocol.
• ZlibDecompressor is immutable but it is a true container (it contains a dict), so it requires the GC protocol.

[picnixz]

In a follow-up, I'll add the immutable flag as well. We still need the GC protocol as the type is not empty (I think? or maybe we don't need it, I'll need to check).